Sulfur substitution in Fe-MOF-74: implications for electrocatalytic CO₂ and CO reduction from an ab initio...

Employing first-principles methods, we investigated the electrocatalytic reduction of CO₂ and CO on two Fe-based MOFs: Fe₂DOBDC and Fe₂DSBDC. Our primary objective was to discern the impact of substituting S atoms into the framework while maintaining the topological structure. We anticipated significant changes in reduction reactions due to differences in chemical properties such as electronegativity, atomic radius, polarizability, and charge density upon replacing O atoms with S atoms. Atomic charge analysis highlights some of these differences by showing the equatorial Fe-O/S bonds of Fe₂DSBDC are less polarized and result in smaller positive and negative charges on the Fe and O/S atoms, respectively. Additionally, the larger S atoms are expected to weaken adsorbate binding due to less favorable van der Waals interactions near the open-metal Fe site. Consequently, the less electropositive Fe site and the larger S atoms of Fe₂DSBDC impede the adsorption of reduced CO₂ and CO products, while the more electropositive Fe site and smaller O atoms of Fe₂DOBDC strongly favor product adsorption. Specifically, the weak binding of HCOOH and CH₂O intermediates on Fe₂DOBDC (ΔG of -0.07 eV and -0.11 eV, respectively) indicates feasible further reduction to CH₂O and CH₄ for CO₂RR and CORR, respectively. Conversely, these adsorbates exhibit unfavorable binding to the Fe site of Fe₂DSBDC (ΔG of +0.14 eV and +0.25 eV, respectively), limiting further reduction possibilities. Thus, CO₂ and CO reduction on Fe₂DSBDC are likely to yield only 2e^- products (HCOOH and CH₂O, respectively), whereas Fe₂DOBDC is expected to produce deeper reduction products (CH₂O and CH₄, respectively). Additionally, significant differences in free energy for the first reduction steps post-sulfur substitution indicate more favorable energetics for both CO₂ and CO reductions (ΔG = -0.12 eV and -0.58 eV, respectively). These findings lay the groundwork for designing novel MOFs with tunable reaction behaviors by strategically replacing O atoms with heavier S atoms in the MOF scaffold.